1. Field of the Invention
This invention relates to a method and apparatus for detecting an abnormal image pattern, wherein an abnormal image pattern, which appears in a reproduced visible image due to dust, a flaw, or the like, located on a surface of a stimulable phosphor sheet, or due to a noise component, is detected from image signals, which are obtained from an image read-out operation carried out on the stimulable phosphor sheet.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal (i.e., an image signal), and the image signal is processed and then used for reproducing the X-ray image as a visible image on a photocopy, or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like, can be reproduced.
Further, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then processed and used for the reproduction of the radiation image of the object as a visible image on a recording material.
As a method for photoelectrically detecting light emitted by a stimulable phosphor sheet, a method for detecting light emitted by two surfaces of a stimulable phosphor sheet has been proposed in, for example, U.S. Pat. No. 4,346,295. With the proposed method for detecting light emitted by two surfaces of a stimulable phosphor sheet, two photoelectric read-out means are located on opposite sides of the stimulable phosphor sheet. The two surfaces or only one surface of the stimulable phosphor sheet is scanned with the stimulating rays, and the light emitted by the two surfaces of the stimulable phosphor sheet is photoelectrically detected by the two photoelectric read-out means. With the proposed method for detecting light emitted by two surfaces of a stimulable phosphor sheet, a single radiation image is stored on the stimulable phosphor sheet, and the light emitted by two surfaces of the stimulable phosphor sheet is detected on the two sides of the stimulable phosphor sheet. Therefore, the efficiency, with which the light emitted by the stimulable phosphor sheet is guided and detected, can be kept high, and a high signal-to-noise ratio can be obtained.
With the method for detecting light emitted by two surfaces of a stimulable phosphor sheet, which has been proposed in U.S. Pat. No. 4,346,295, a stimulable phosphor layer is overlaid on a surface of a transparent substrate, and a stimulable phosphor sheet is thereby formed. The stimulable phosphor sheet, on which a radiation image has been stored, is placed on a transparent holder, and two photoelectric read-out means are respectively located above and below the holder. Specifically, the light emitted from the front surface of the stimulable phosphor sheet is detected by the photoelectric read-out means, which is located above the holder. Also, the light emitted from the back surface of the stimulable phosphor sheet is detected by the photoelectric read-out means, which is located below the holder.
When a radiation image is recorded on a recording media, such as a stimulable phosphor sheet or X-ray film, if dust clings to the surface of the recording media, the radiation irradiated to the recording media will be blocked by the dust. Therefore, when a visible image is reproduced from the image signal, which has been obtained from the recording media carrying a radiation image thus recorded, a white dot-like pattern occurs as an artifact in the reproduced visible image at a portion corresponding to the part of the surface of the recording media, to which the dust clung. Also, in particular, in the radiation image recording and reproducing system in which a stimulable phosphor sheet is used, when the stimulable phosphor sheet, on which the radiation image has been stored, is exposed to stimulating rays, and the radiation image is thereby read out from the stimulable phosphor sheet, the stimulating rays are blocked by dust clinging to the stimulable phosphor sheet. As a result, a white dot-like pattern occurs in the reproduced visible image. Further, in the radiation image recording and reproducing system, if flaws are present on the surface of the stimulable phosphor sheet, the stimulating rays will be scattered by the portion at which the flaws are present. As a result, a white dot-like pattern occurs in the reproduced visible image.
In cases where the reproduced visible image is used in making a diagnosis of an illness of a human body, or the like, the aforesaid white dot-like pattern has very detrimental effects. Specifically, for example, in cases where a radiation image of the mamma, or the like, is recorded and reproduced, a calcium salt, which has deposited in a living body, appears as a white dot-like pattern, and the white dot-like pattern due to the deposition of the calcium salt (i.e. the calcification) is utilized in finding a cancer, or the like. Therefore, if the white dot-like pattern due to the calcification and the white dot-like pattern due to dust or flaws described above cannot be discriminated from each other, and an error will occur in making a diagnosis. The sizes of the white dot-like patterns due to dust or flaws will vary in accordance with the sizes of the dust or the flaws. However, actually, it often occurs that the sizes of the white dot-like patterns due to dust or flaws are approximately equal to the sizes of white dot-like patterns due to the calcification.
In order for the aforesaid problems to be prevented from occurring, the clinging of dust to a recording medium or the occurrence of flaws in the recording medium have heretofore been prevented with various methods. For example, a method for removing dust, which has clung to a stimulable phosphor sheet, by using a cleaning roller has been proposed in, for example, U.S. Pat. No. 4,703,537.
Also, a method for detecting an abnormal image pattern has been proposed by the applicant in U.S. Pat. No. 5,231,574. With the proposed method, specific image signal components, which take values smaller than a predetermined threshold value, are found from an image signal, which is made up of a series of image signal components representing picture elements in an image. Thereafter, the number of picture elements, which are adjacent to one another and which the specific image signal components represent, is counted. In cases where the number of such picture elements is not larger than a predetermined value, it is regarded that the specific image signal components are artifact signal components. In this manner, a white dot-like pattern due to the calcification and a white dot-like pattern, which is an artifact due to dust, flaws, or the like, are discriminated from each other.
Stimulable phosphor sheets have a very high sensitivity. Therefore, a stimulable phosphor sheet also stores energy from radiation delivered by radioactive isotopes, such as .sup.226 Ra and .sup.40 K, which are contained in a trace amount in the stimulable phosphor of the stimulable phosphor sheet, or energy from environmental radiation, such as cosmic rays and radiation delivered by radioactive isotopes, which are contained in paints on indoor walls, or the like. In cases where the operations for recording and reproducing a radiation image are carried out by use of the stimulable phosphor sheet, on which energy from such radiation has been stored, a black dot-like pattern will occur in a reproduced visible image. The black dot-like pattern adversely affects the image quality of the reproduced visible image.
If a noise component, which forms the aforesaid black dot-like pattern, can be detected from the image signal obtained from the aforesaid radiation image read-out operation, it will become possible to prevent the black dot-like pattern from occurring in the reproduced visible radiation image by carrying out various processes on the image signal in order to eliminate the noise component.
Accordingly, in U.S. Pat. No. 4,920,267, the applicant proposed a method for detecting noise in image signals. The proposed method comprises the steps:
i) comparing the value of an image signal component a, which represents each remark picture element A, with values (b.sub.1 +T.sub.1), (b.sub.2 +T.sub.2), . . . , (b.sub.n +T.sub.n), which are obtained by adding predetermined values T.sub.1, T.sub.2, . . . , T.sub.n respectively to the values of image signal components b.sub.1, b.sub.2, . . . , b.sub.n representing a plurality of picture elements B.sub.1, B.sub.2, . . . , B.sub.n located in the vicinity of the remark picture element A, and PA1 ii) recognizing that noise is contained in the image signal component a in the case where the image signal component a takes a value larger than every one of the values (b.sub.1 +T.sub.1), (b.sub.2 +T.sub.2), . . . , (b.sub.n +T.sub.n). In this manner, a noise component is accurately detected from the image signal, which has been obtained from the radiation image read-out operation carried out on a stimulable phosphor sheet. PA1 i) subtracting the image signal components of the two image signals from each other, which image signal components represent corresponding picture elements on one of the two surfaces of the stimulable phosphor sheet and the other surface of the stimulable phosphor sheet, difference values being thereby obtained, PA1 ii) making a judgment as to whether each of the difference values falls or does not fall within a predetermined range, and PA1 iii) in cases where a difference value is judged as being outside the predetermined range, determining that an abnormal signal component is contained in at least either one of the two image signals. PA1 i) a difference value calculating means for subtracting the image signal components of the two image signals from each other, which image signal components represent corresponding picture elements on one of the two surfaces of the stimulable phosphor sheet and the other surface of the stimulable phosphor sheet, difference values being thereby obtained, and PA1 ii) a judgment means, which makes a judgment as to whether each of the difference values falls or does not fall within a predetermined range and which, in cases where a difference value is judged as being outside the predetermined range, determines that an abnormal signal component is contained in at least either one of the two image signals.
Also, in cases where the operation for detecting light emitted by two surfaces of a stimulable phosphor sheet is carried out, such that an image free of noise and having good image quality may be obtained, it is necessary to detect an abnormal image pattern. Therefore, it is considered to apply the method proposed in U.S. Pat. No. 4,920,267 or 5,231,574.
However, with the operation for detecting light emittedby two surfaces of a stimulable phosphor sheet, two image signals are obtained respectively from the two surfaces of the stimulable phosphor sheet. Therefore, it is necessary for the operation for detecting an abnormal image pattern to be carried out two times for a single radiation image stored on the stimulable phosphor sheet. Accordingly, considerable time and labor are required to detect an abnormal image pattern and to carry out the processes for eliminating noise due to the abnormal image pattern.